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Date: Fri, 13 Nov 2020 13:07:21 +0200 From: Adrian Ratiu <adrian.ratiu@...labora.com> To: Ard Biesheuvel <ardb@...nel.org> Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>, Nathan Chancellor <natechancellor@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>, Arnd Bergmann <arnd@...db.de>, Russell King <linux@...linux.org.uk>, Arvind Sankar <nivedita@...m.mit.edu>, kernel@...labora.com, clang-built-linux <clang-built-linux@...glegroups.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning Hi Ard, On Fri, 13 Nov 2020, Ard Biesheuvel <ardb@...nel.org> wrote: > On Thu, 12 Nov 2020 at 22:23, Adrian Ratiu > <adrian.ratiu@...labora.com> wrote: >> >> From: Nathan Chancellor <natechancellor@...il.com> >> >> Drop warning because kernel now requires GCC >= v4.9 after >> commit 6ec4476ac825 ("Raise gcc version requirement to 4.9"). >> >> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com> >> Signed-off-by: Nathan Chancellor <natechancellor@...il.com> >> Signed-off-by: Adrian Ratiu <adrian.ratiu@...labora.com> > > Again, this does not do what it says on the tin. > > If you want to disable the pragma for Clang, call that out in > the commit log, and don't hide it under a GCC version change. I am not doing anything for Clang in this series. The option to auto-vectorize in Clang is enabled by default but doesn't work for some reason (likely to do with how it computes the cost model, so maybe not even a bug at all) and if we enable it explicitely (eg via a Clang specific pragma) we get some warnings we currently do not understand, so I am not changing the Clang behaviour at the recommendation of Nick. So this is only for GCC as the "tin" says :) We can fix clang separately as the Clang bug has always been present and is unrelated. > > Without the pragma, the generated code is the same as the > generic code, so it makes no sense to build xor-neon.ko at all, > right? > Yes that is correct and that is the reason why in v1 I opted to not build xor-neon.ko for Clang anymore, but that got NACKed, so here I'm fixing the low hanging fruit: the very obvious & clear GCC problems. >> --- >> arch/arm/lib/xor-neon.c | 9 +-------- >> 1 file changed, 1 insertion(+), 8 deletions(-) >> >> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c >> index b99dd8e1c93f..e1e76186ec23 100644 >> --- a/arch/arm/lib/xor-neon.c >> +++ b/arch/arm/lib/xor-neon.c >> @@ -19,15 +19,8 @@ MODULE_LICENSE("GPL"); >> * -ftree-vectorize) to attempt to exploit implicit parallelism and emit >> * NEON instructions. >> */ >> -#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6) >> +#ifdef CONFIG_CC_IS_GCC >> #pragma GCC optimize "tree-vectorize" >> -#else >> -/* >> - * While older versions of GCC do not generate incorrect code, they fail to >> - * recognize the parallel nature of these functions, and emit plain ARM code, >> - * which is known to be slower than the optimized ARM code in asm-arm/xor.h. >> - */ >> -#warning This code requires at least version 4.6 of GCC >> #endif >> >> #pragma GCC diagnostic ignored "-Wunused-variable" >> -- >> 2.29.2 >>
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